Static mass spectrometers are used when the highest possible degree of sensitivity is required. Analysis is typically conducted for detecting the presence of minute quantities of noble gases (He, Ne, Ar, Kr, Xe), but they may also be capable of analyzing gases such as CO2 or N2.
The operation of static mass spectrometers exhibits several special features. A characteristic feature of static mass spectrometers is that they stay evacuated but are not pumped during analysis.
The main components of a static mass spectrometer comprise an ion source, an analyzer, an ion detector and a pump for generating a high vacuum in the mass spectrometer. Operations commence with the generation of a high vacuum in the mass spectrometer. Then the mass spectrometer is disconnected from the pump (normally by means of a valve) and minute quantities of the gas to be analyzed are admitted to the mass spectrometer.
Referring to FIG. 1, there is shown a typical schematic configuration of an existing static mass spectrometer 200, comprising: a sample preparation region 205; a transfer region 230; an ion source region 240; and a mass analyzer 250. The sample preparation region 205 comprises a furnace 210 and an optional preparation bench 220. Between each of the furnace 210, the sample preparation bench 220, the transfer region 230 and the ion source region 240, valves 215 are provided.
The admission to the static mass spectrometer 200 is indirect via an intermediate chamber. A normal application is the determination of the isotope ratios of various isotopes of a noble gas that is trapped in a sample, such as a piece of rock or similar.
In current instruments, the sample, typically a piece of rock, is put into a chamber (such as furnace 210) and then heated, possibly with a laser. This treatment releases trapped gases, which comprise the desired analytes. The released gases are transferred to the sample preparation bench 220, where they may be manipulated in various ways. For example, they may be partially or wholly transferred to storage volumes (“pipettes”) and then they may be partially released, giving a smaller amount of sample at a lower pressure.
The gas is then transferred to the transfer region 230, which may act as a cleaning unit. In older devices, the gas was collected on a cold finger. More modern device comprise a general type of “trap” installed, typically comprising chemical getters to remove unwanted substances (this usually means everything but noble gases). The getters are cryo-cooled and may be thawed to “distill” the gases, releasing them one after another. From this moment onwards, the pumps are closed off by valves before the sample is released into the chamber (240, 250).
From here, the gas is thawed and equilibrated with the ion source region 240 where the gas is ionized (Electron Ionization) and the ions are subsequently analyzed in the mass analyzer 250. The noble gas need not always be frozen (and then thawed), for example the lighter gases such as helium and neon which are difficult to freeze. Then, the noble gas would pass straight to the ion source region 240 with just the impurities being frozen out in the transfer region. In such embodiments, the gas to be analysed is equilibrated with the ion source after the transfer region 230.
In the ion source 240, the gas to be analyzed is typically ionized by means of electron bombardment. Due to the statistical distribution in the mass spectrometer of the gas to be analyzed, there are only a small number of molecules in the region of the ion source. This therefore results in only a small ion stream.
Typical pressures in the ion source region 240 and mass spectrometer 250 are 10−9 to 10−10 mbar before the sample is admitted and subsequently, 10−6 to 10−7 to 10−9, depending on the sample amount (which cannot always be predicted). The gas to be analyzed spreads throughout the ion source region 240 and the mass analyser 250, with a few molecules also entering the ion source. In the mass analyser 250, the ions from the ion source travel along a flight tube 255 before being detected in detector region 260.
The strong vacuum and the removal of “uninteresting” gases from the sample are very desirable to improve the signal to noise ratio (that is the ion count from the sample against the ion count from other gases remaining from a previous measurement or other “interferences”, such as isobaric ions like hydrocarbons).
In static mass spectrometry, the interior free volume for the gas becomes a major performance parameter. The sensitivity is directly proportional to the interior volume, such that the larger the volume of the instrument, the lower the sensitivity. Similarly large surfaces are feared as sources of contamination as well as potential places for sample to settle on, leading to reduction of sensitivity and possibly memory effects (of the type noted above that might affect the signal to noise ratio). However, reducing the volume normally results in a reduction in the distance between high voltage parts of the ion source and the grounded source housing. This significantly increases the risk of undesirable current discharge from the ion source. A high potential is required to effect ionization, whereas the housing defining most of the ionization volume dimensions is usually grounded leading to the risk of sparking.